1. Technical Field
This invention relates to a method and apparatus for sensing inclusions in molten metal.
2. Background
Inclusions in molten or liquid metal are impurities found in the liquid metal. The presence of inclusions can affect the quality of ingots cast from that liquid metal. The quality of the ingot affects the quality of the subsequent sheet and plate products fabricated from that ingot.
High concentrations of inclusions in molten or liquid metal, e.g., such as inclusions in molten aluminum typically at sizes of 15 to 120 microns and in amounts over about three kilocounts per kilogram, adversely affect metal quality.
The impurities forming the inclusions in the liquid metal, in contrast to elemental impurities, are particles of separate phases suspended in the metal.
The inclusions in the liquid metal can be classified into one of two types of inclusions. The two types are hard and soft inclusions.
Hard inclusions primarily are attributable to oxides or other non-deformable particles. Examples of hard inclusions include magnesium oxide, aluminum oxide, spinel (magnesium aluminum oxide or MgAl2O4), silicon oxide, aluminum carbide, silicon carbide, titanium diboride, and vanadium diboride in certain circumstances.
The hard inclusions either come from the metal source itself, whether from ore or recycled metal, when it gets melted, or are created by oxidation during the melting process or elsewhere in the process because of oxygen present in the air or because of water vapor present, because of cascading or turbulence in the furnace, e.g., because of a fixed tap hole making for a lot of turbulence in the metal at the furnace outlet as liquid metal flows into the trough.
Oxide inclusions arise from the oxidation of aluminum or aluminum alloy at some point in its processing. Melting of scrap metal can result in oxide films, formed on the surface of the aluminum, being entrained in the molten metal. Alloying molten aluminum with magnesium can produce magnesium oxide inclusions from oxidation of the magnesium. Oxidation of molten metal as it flows through the launder system also can produce oxide inclusions. Turbulence at the molten metal surfaces and cascading of molten metal through air promote the formation of inclusions.
Aluminum carbide inclusions arise from the Hall-Heroult electrolytic cells in which primary aluminum is produced. Aluminum reacts with carbon cathodes in these cells to form aluminum carbide.
Silicon oxides arise from refractories used in furnaces, launder systems, and casting spouts.
Boride inclusions arise from grain refiners used to control the grain size in the solidified metal. Inclusions of borides would be a much larger size than boride particles that are effective as nucleating sites for aluminum grains.
Soft inclusions are attributable to molten salt droplets, gas bubbles, agglomerates of other very small particle types, or other deformable inclusions. Examples of soft inclusions include magnesium chloride, sodium chloride, potassium chloride, calcium chloride, aluminum chloride, cryolite salts, calcium fluoride, and liquid solutions containing these compounds or a combination of these compounds. Agglomerates of borides are another example of soft inclusions.
The soft inclusions come from using chlorine and chloride to treat the metal, e.g., in an in-line degassing process wherein even small amounts of chlorine gas form aluminum chloride and magnesium chloride. The aluminum chloride is not stable and tends to react to form magnesium chloride.
Fluxing molten metal with chlorine or a combination of an inert gas with chlorine removes hydrogen, alkali metals, alkaline earth metals, and hard inclusions from the molten metal.
Chlorine is used to react chemically with alkali and alkaline earth metals. The chlorine aids wetting of hard inclusions by flux gas bubbles and allows for removal by flotation. The chlorine aids the separation of skim from molten metal. The formation of molten chloride inclusions is a by-product of using chlorine to treat the metal. Fluxing is carried out in furnaces and in-line during the casting process.
The soft inclusions also come from using granular salts in the furnace, e.g., magnesium chloride, calcium chloride, sodium chloride, and potassium chloride, and combinations of magnesium chloride, calcium chloride, sodium chloride, and potassium chloride. These salts are used for reacting to remove sodium and calcium from the metal, to minimize melt loss, or to keep the furnace clean. Such granular salts in the furnace cause some carryover of molten salt inclusions.
The hard inclusions and soft inclusions range in size from about 1 micron to several hundred microns.
The total concentration of hard inclusions and soft inclusions is about 0.05-0.1 kilocount per kilogram, i.e., 50-100 particles per kilogram of metal, to about 150 kilocount per kilogram, i.e., 150,000 particles per kilogram of metal. The total concentration of hard inclusions and soft inclusions depends on the source of the metal from scrap type, solid primary metal, molten primary metal, or remelted beverage cans. The total concentration of hard inclusions and soft inclusions depends on settling time allowed in the furnace, the cleanliness of the furnace, the in-line treatment of the metal, the in-line filtration of the metal, and the design of the launder system.
Inclusions cause problems which depend on the type of product and the gauge of the product. For example, inclusions affect three inch plate differently from 6 micron thick foil. The inclusions will cause a hole or pinhole in the 6 micron thick foil. In beverage can or food can sheet, for example, pinholes are serious concerns.
Inclusions cause pinholes in foil and rigid container sheet such as food can sheet or beverage can sheet. Inclusions cause breakage of wire during drawing operations. Inclusions cause surface imperfections such as streaking in bright products such as reflector sheet or automobile trim. Inclusions also cause surface defects during extrusion processes. Inclusions also serve as nucleating sites for the formation of gas bubbles during solidification and thereby affect the fatigue life of certain products.
The two different inclusion types affect metal quality in different ways. It is difficult to know whether hard inclusions or soft inclusions cause particular problems because of an inability to distinguish between different inclusions.
Inclusions in the metal are analyzed by destructive methods of analysis. Currently, inclusions in the metal are analyzed and classified by destructive testing by taking a sample of the metal, solidifying the metal sample, cutting open the solid metal, looking at it under a microscope, and classifying or identifying the inclusion in the solid metal sample to determine qualitatively, and semi-quantitatively, whether the inclusion is a hard or soft inclusion.
Currently, inclusions in the molten metal are analyzed and classified by destructive testing by taking a sample of the molten metal and metallographically analyzing the sample for inclusions. The inclusions in the metal are concentrated in the sample by passing the molten metal through a filter or frit and then metallographically analyzing the sample to search for the inclusions at the leading edge of the filter or frit. Podfa and LAIS are trade marks of two commercially available sampling systems based on metallographic analysis. The metallographic analysis identifies the inclusion types and distinguishes between what are hard and soft inclusions at molten metal temperatures. However, the metallographic analysis is only semi-quantitative and does not provide results in real time.
Currently, inclusions in the metal are analyzed by non-destructive testing by ultrasonic testing. However, ultrasonic testing is performed only on the metal after it is solidified into a solid part. Moreover, ultrasonic testing provides a resolution of only about {fraction (1/64)} inch. Much smaller inclusions cause problems in certain products. For example, a six micron diameter inclusion will cause a pinhole in 6 micron thick foil.
It is difficult to know from ultrasonic non-destructive testing on a solidified part whether an inclusion originated as a hard or soft inclusion. For example, it is sometimes difficult to know with certainty whether a processing problem is caused by a hard inclusion or hydrogen. In some cases, the inclusion may serve as nucleating sites for hydrogen bubbles. It would be highly advantageous to know, in real time, the identification and classification of the inclusion.
Current instrumentation technology for measuring inclusions in liquid metal streams does not discriminate between the two different types of inclusions. The current instrumentation technology does not identify whether the inclusions are hard or soft inclusions. The current method detects the existence of any inclusions and provides aggregate measures of inclusion concentrations and particle size.
Lumping together of inclusion types is undesirable because process operators and engineers prefer to know the type of inclusions in the liquid metal to determine the potential sources for the contamination and to determine ways to reduce or eliminate the harmful inclusions. Because the two inclusion types affect metal quality in different ways and because the two inclusion types are controlled in different ways, a method and apparatus are needed to discriminate and identify the two different types of inclusions.
A method and apparatus are needed to discriminate and classify or identify the two different types of inclusions in liquid metal streams in real time.
The ability to classify particle types in liquid metal streams in real time would enable the creation of guidelines on what to change in a continuous casting process in real time. The ability to classify particle types in liquid metal streams in real time would enable the creation of procedures to eliminate or reduce the influence of the inclusions on ingot quality in real time. For example, if a preponderance of salt particles (i.e., soft inclusion) is detected in real time, the guideline may be to reduce a chlorine rate in an on-going continuous casting process to optimize salt collector, to reduce a flux amount, or to add a filter. Improving the quality of cast ingot in this way would greatly improve the processing of products fabricated downstream from the continuous casting process.
A current instrument used for measuring inclusion concentrations employs a Coulter counter as a liquid stream passes through an orifice, using counter principles for its sensing element. By Coulter counter is meant a technique for counting pulses as a liquid stream passes through an orifice. A constant current is passed between electrodes on both sides of the orifice. As inclusion particles are drawn through a small orifice, and as a voltage between the electrodes increases, the electronic sensor produces exponentially-shaped voltage pulses. The voltage pulses have amplitudes which are a function of the effective particle diameter.
U.S. Pat. Nos. 4,555,662 and 4,600,880 to Doutre et al. disclose a method and apparatus, known as LiMCA for Liquid Metal Cleanliness Analyzer, for the detection of non-conductive particulates in molten aluminum, gallium, zinc and lead. A very small diameter passage into the container (about 300 micrometers for aluminum) forms a current path between two electrodes carrying a current of up to 500 amperes. The path is surrounded by liquid metal which forms a Faraday cage screening the path, enabling the passage of a particulate of about 15 micrometers or larger to produce a voltage pulse between the electrodes of greater than 5 microvolts, which is detectable above the background noise, which is of about that value.
U.S. Pat. No. 4,763,065 to Hachey discloses an apparatus for the detection and measurement of suspended particulates in a molten metal. A container composite wall includes concentric electrically conducting outer and inner walls (10) and (12) and a disc (14) of refractory material having a passage (16) of predetermined size. Molten metal is pumped through the passage (16) to establish a current path from the inner wall through the passage to the outer wall. A current is passed along the current path, and voltage changes are measured as indicating passage of suspended particulates through the passage.
U.S. Pat. No. 5,039,935 to Hachey discloses on-line particle determination in molten metals. A sample of molten metal is drawn by a vacuum through a calibrated passage in the side wall of a heat resistant tube while a current is established through the passage between two electrodes. The passage of non-conducting particles through the orifice produces pulses of magnitude and rate indicate respectively their size and the number of particles per unit volume. The electrodes and their leads form an interference-receiving antenna so that the wanted test signal, whose signal/noise ratio is inherently low, is subject to interference from neighboring sources, such as motors, fluorescent lamps and particularly induction furnaces. The patent discloses a cancellation signal to reduce the unwanted interference component in the test signal by a cancellation antenna.
U.S. Pat. No. 5,130,639 to Hachey discloses an apparatus for on-line particle determination in molten metals. A sample of molten metal is drawn through a calibrated passage while a steady current is established through the passage between two electrodes. Particles moving through the passage produce voltage pulses at a magnitude and rate to indicate size and number per unit volume. The wanted test signal is obtained between two current-carrying electrodes, or between two other electrodes disposed on opposite sides of the passage. The electrodes form an interference-receiving antenna so that the wanted low signal/noise ratio test signal is overlaid with interference from neighboring sources. This interference is reduced by a cancellation signal produced by a cancellation antenna constituted by a similar pair of electrodes, either separate from the main current-carrying electrodes or having one of them in common. A four electrode configuration permits the diameter of the passage to be monitored continuously. A five electrode configuration has three electrodes separate from the current-carrying electrodes forming the two antennae and connected differentially. A further electrode minimizes ground loops in the signal path. A head member carrying the tube and electrode cluster is mounted to move between operative and storage positions relative to a main body member carrying the power supply; these are made symmetrical about a longitudinal axis so that interference signals are in anti-phase and cancel.
U.S. Pat. No. 5,241,262 to Guthrie et al. discloses a molten metal inclusion sensor intended for xe2x80x9ccontinuousxe2x80x9d use in the testing of steel, i.e., a useful life of at least about 30 minutes. A probe detachably connected to a water-cooled support member (35) includes a tube (30) of heat resistant material, preferably silica, an inner electrode (31) mounted on its interior wall, and an outer electrode (32) mounted on its exterior wall. Molten metal enters the tube interior through an orifice (33) upon its immersion in the molten metal. The flow of metal with entrained inclusions is monitored by measuring the voltage between the electrodes (31, 32). The electrodes (31, 32) are preferably of graphite and are shaped to fit closely against the walls of the part of the tube (30) immersed in the metal and are of a material that retains enough mechanical strength to support the tube (30) as the metal is pumped into and out of the interior, the metal remaining hot enough for this pumping to occur. The orifice (33) is contoured to produce streamline flow and the Reynolds number of the flow preferably is kept below 2000.
U.S. Pat. No. 5,584,578 to Clauss, Jr. discloses a drop-in immersion probe for inserting into molten metal. A cylindrical measurement head has an axis and a first axial end inwardly tapered toward the axis. The measurement head is made of a combination of materials having a combined density greater than the density of the molten metal. A sensor element extends outwardly from the first axial end of the measurement head proximate the axis, and a slag cap covers the first end of the measurement head and the sensor element. Lead wire extends outwardly from the measurement head and has one end electrically connected to the sensor element. A portion of the lead wire extending outwardly from the measurement head is covered by a protective sleeve of heat-resistant material.
U.S. Pat. No. 5,198,749 to Guthrie et al. discloses a molten metal inclusion sensor. A single use disposable probe tube of heat resistant material has an inner electrode mounted on its interior wall and an outer electrode mounted on its exterior wall. The molten metal enters the tube interior through an orifice in its wall past a jet-preventer insert upon its immersion in the molten metal. The flow of metal with entrained inclusions is monitored by measuring the voltage between the electrodes. The Reynolds number of the flow is maintained below 2000. The tube interior is divided by a narrow bore into two compartments so that metal enters one compartment and freezes in the bore so that it cannot enter the second compartment, protecting the vacuum source and establishing the quantity of metal entering the probe. The orifice is closed by a meltable cover, and the cover is protected by a meltable shield to enable the probe to be passed through an overlying slag layer without entry of slag to the probe interior. The electrodes and the body of the probe should have a useful life in the bath of about 2 minutes.
It is an object of the present invention to discriminate and identify different types of inclusions in a liquid metal stream.
It is an object of the present invention to discriminate and identify different types of inclusions in a liquid metal stream in real time.
It is another object of the present invention to discriminate and identify between hard inclusions and soft inclusions in a liquid metal stream in real time.
It is another object of the present invention to discriminate and identify between hard inclusions and soft inclusions in a liquid metal stream in real time.
It is an object of the present invention to provide a realtime DSP (Digital Signal Processing) based pulse classification and peak detection method and apparatus for separating two distinct signal pulse classes from an analog signal stream.
It is an object of the present invention to provide updated histograms of the current distribution of amplitudes related to inclusion size, to maintain counts of each detected pulse type, and to compute a running probability of occurrence value for each of the pulses.
It is an object of the present invention to provide a displays and controls graphical user interface GUI screen to summarize data processed by the details of the algorithms and to present a visual display of the summarized data in a suitable operator interface.
These and other objects of the present invention will become apparent from reference to the figures of the drawings and the detailed description which follow.
The present invention provides an apparatus and method of determining a classification of inclusions in molten metal. The apparatus and method of the present invention include the steps of and means for obtaining an analog signal stream from a LiMCA data collection apparatus, passing the analog signal stream through an analog to digital converter to convert the analog signal stream to a digital signal stream, and partitioning the digital signal stream into discrete time frames or vector of 5 milliseconds length and sampled at a rate of 10 kHZ. The digital signal stream is normalized to provide a normalized signal vector, preferably, having a largest vector magnitude of 1. The normalized signal vector is compared to a control prototype shape for determining a classification for the digital signal vector over the discrete time frame by decision logic to determine hard inclusions versus soft inclusions in the liquid metal. In one aspect, the normalized signal vector is passed through a decision module having at least one threshold unit and at least one digital logic table to make a decision of a soft, deformable inclusion or a hard inclusion. In one embodiment of the present invention, the classification and size of the inclusions are counted in an histogram of classification and a histogram of size. The classification and size of the inclusions are viewed in a graphical user interface.